Image Compression and Batch Processing

PNG8 Palette-Based Alpha Transparency

Web designers use alpha channel to create various effects (like translucent surfaces,
shadow and glow effects) and for edge anti-aliasing. Starting from the version 2.4.3,
JPEG Imager has an ability to emulate alpha channel
semi-transparency for palette-based PNG files.

PNG format specification allows defining more than one palette entry as fully or
partially transparent. This feature of the format makes it possible to convert a
32-bit true color image to 8-bit
indexed palette while preserving transparency information. Converting images to
indexed color format may help to optimize page loading time since converted files
are usually much smaller.

Modern web browsers, such as Internet Explorer 7-9, Mozilla Firefox, Opera 6+ and Safari
have full alpha support (including palette alpha). But earlier versions of Internet
Explorer (5.5-6.x/Win32), which are still in wide use today, will handle true alpha
channel incorrectly, rendering non-palette images fully opaque with a solid gray
background by default.

The indexed alpha images, however, are displayed differently in those older browsers,
as if the image has a single transparent color (binary transparency, similar to
GIF). Such limited transparency often produces a better effect (compared to absolute
opaqueness) with acceptable degradation of visual quality.

Examples, 200% zoom

PNG32 rendered in Firefox

PNG32 rendered in IE6

PNG8 rendered in Firefox

PNG8 rendered in IE6

This unique property of PNG8 makes it more "compatible" with crippled old browsers,
yet enables full power of alpha transparency in those browsers that support it.

Most of the image-editing tools are unable to create palette-based images with variable
transparency. You can use JPEG Imager to convert your graphics to PNG8
format. JPEG Imager implements high quality color quantization algorithm
with real-time preview. It efficiently reduces the number of colors in the image
taking into account alpha values. See a sample picture at the bottom of this page.
You also have an option to create images with old-fashioned binary transparency
with dithered alpha.

Transparency Demo

click on the pictures below to change their background (java-script required)